The requirement of supplying electrical energy to drive electrochemical reactions often makes such reactions cost prohibitive. For this reason, use of solar energy to power electrochemical reactions has long been contemplated. The electrochemical reactions most often so considered include the decomposition of water into the gases hydrogen and oxygen, and the degradation of organic contaminants from an effluent stream.
The conversion of sunlight and water into a clean, high efficiency chemical fuel has been a goal for a number of years and the urgency increases as damaging effects of burning fossil fuels becomes ever more apparent. Photolysis of water to yield hydrogen and oxygen has been widely demonstrated under visible light illumination of Group II-sulfides and selenides (i.e. CdS, CdSe, ZnS, etc.), and under ultraviolet light illumination of TiO.sub.2, BaTiO.sub.3, ZnO, etc. These semiconductors are typically loaded with metal and or metal oxide catalysts to promote the transfer of electrons and holes, respectively to the reactant-device interface.
The use of solar energy to decompose contaminants such as organics, inorganic salts and microbes allows for remediation to effectively cope with seepage of pollutants into an environment. The photocatalyzed decomposition of organic contaminants is well known to the art(K. Rajeshwar, J. of Applied Electrochemistry 25, 1067(1995)). A variety of halocarbons react under exposure to ultraviolet light in the presence of TiO.sub.2 to give less noxious byproducts (C-Y. Hsiao, J. of Catalysis 82, 412 (1983)). Other transition metal oxides and Group II-VI semiconductors have also been utilized to degrade environmental contaminants.
In general, current photocatalytic systems suffer from low reaction rates. Reaction-induced changes in pH, donor concentrations and surface trap sites are at least partly responsible for the low reaction rates observed. In the case of some of the Group II-VI semiconductors, the exposure to light and the presence of oxygen digests the semiconductor domain. For example, while CdS has favorable light absorption properties, under aqueous reaction conditions it is converted to the water soluble compound CdSO.sub.4, thereby consuming the semiconductor in the course of reaction. Reaction rates suffer further from harvesting only those incident photons which are greater than the band gap absorption of the semiconductor. Thus, it is an object of the present invention to provide a semiconductor that has a tunable band gap such that harnessable light energy is collected efficiently in the course of photochemical reaction and with minimal changes in efficiency with operation.
Platinized n-type bulk silicon has proven to be an inefficient photocatalyst for hydrogen generation from organic molecules. The band gap of silicon was identified as being less than the theoretical decomposition voltage of the half-cell reactions (H. Yoneyama, N. Matsumoto and H. Tamura, Bull. Chem. Soc. Jpn. 59, 3302 (1986)). Thus, it is a further object of the present invention to provide a silicon semiconductor with a more favorable band gap for photocatalytic decomposition reactions than that of bulk silicon.
Another object of the present invention is to provide a device for the selective photoelectrolysis of water, inorganic salts and organic molecules that shows superior stability towards the corrosive solution environment as compared to binary and ternary semiconductor compounds.
Still another object of the present invention is to provide a method for producing hydrogen and oxygen photocatalytically from water.
The stability of silicon oxides, the tunable control of energy gap and charge carrier characteristics affords heretofore unobtainable photolysis rates over time for catalyzed, quantum confined silicon particles towards water and various aqueous pollutants.
Other and further objects, features and advantages of the present invention will become apparent in the following description.